Heretofore, vacuum freeze-drying methods have been used in which a solution or a dispersion liquid is sprayed in a vacuum ambience, and a dried solute in the solution or a dispersed material in the dispersion liquid is obtained. In a conventional spray type vacuum freeze-drying apparatus, since it takes a long time to completely sublimate water and completely dry the solute or the dispersed material, the drying time period has been sought to be shortened.
Furthermore, if the temperature around a conveyor tray is raised to be higher than a freezing point of the solution or the dispersion liquid in order to shorten the drying time period, there is a possibility that the objects to be dried, which have been frozen, melt and adhere to each other.
The present invention shortens the time period required in the drying step and prevents the adhesion of the objects to be dried.
A vacuum freeze-drying apparatus is disclosed in Japanese Patent Publication No. 2006-177640, for example.